Biaxially stretched polyester bottles have heretofore been known being obtained by biaxially stretch-blow-forming the performs made of a polyester resin such as a polyethylene terephthalate, and have been widely used as containers for containing a variety of kinds of beverages, foods, liquid detergents and the like owing to their excellent transparency, surface luster, shock resistance and gas barrier property.
Biaxially stretched polyester bottles used for containing beverages, usually, have an average thickness of about 0.25 to 0.30 mm. From the standpoint of lowering the cost and decreasing the weight, however, it has been urged to further decrease the amount of the resin that is used.
From the above viewpoint, JP-A-7-257534 proposes a bottle-like container obtained by blow-forming which is so effected that the average thickness of the body wall is not larger than 0.25 mm and the average stretching ratio is not smaller than 10 times.
The weight of the bottle-like containers can be decreased when they have capacities as great as, for example, 5000 mL. However, it becomes difficult to decrease the weight when the containers have capacities which are as relatively small as 2000 mL or less. In the method of producing the bottle-like containers, further, the average stretching ratio is set to be not smaller than 10 times in the biaxial stretch-blow molding but the longitudinal stretching ratio is in a range of 2 to 2.7 times in a customary manner. If it is attempted to further increase the longitudinal stretching ratio, there occur such problems as whitening due to over-stretching, exfoliation of the layers (delamination) and rupture (burst), etc. So, there comes a problem of molding difficulty.
Further, JP-A-2001-122237 discloses a small-thickness bottle capable of decreasing the weight by decreasing the thickness of part of the container. According to this prior art, the average thickness of the shoulder portion of a bottle of about 500 mL is selected to be 0.2 to 0.3 mm, the average thickness of the body wall is selected to be 0.02 to 0.05 mm, and a transverse rib is formed on the body wall to enhance the strength against the deformation. However, when a bottle having such a large change in the thickness is to be stretch-blow-formed, problems arouse in that the thickness must have been varied in advance when the preform is to be injection-formed and difficulty is in involved in setting the conditions for stretch-blow-forming the preform.
JP-A-2003-191319 proposed by the present inventors discloses a biaxially stretch-blown polyester bottle having an average thickness which is decreased down to 0.1 to 0.2 mm. According to this prior art, a self-standing small-thickness bottle-like container is formed without whitening that is caused by over-stretching, making it possible to decrease the amount of the resin that is used and, hence, to decrease the weight.
However, when the stretch-blow forming is to be effected relying on the production methods described in the above prior arts, there often occurs the burst in the initial stage of blow-forming or a ring-like thick portion is formed if the burst does not occur leaving room for improving the yield of the products.
That is, in the stretch-blow molding employed by the above prior arts, the preform is stretched in the longitudinal direction by a stretching rod prior to blowing the air as shown in FIG. 1(B). It therefore becomes probable that the preform comes in contact with the stretching rod. Besides, as shown in FIG. 1(C), when the preform is forcibly stretched in a state where it can be little stretched since its temperature has dropped upon coming in contact with the surface of the mold on the way of being stretched, there occurs the burst or a parting line is formed, which, then, becomes a cause of burst. Further, even when there does not occur the burst, there occurs a problem in that a portion where the temperature has dropped remains as a ring-like thick portion.